1. Field of the Invention
This invention relates to an axial acceleration sensor particularly suited to detection of pinging of internal combustion engines.
2. Related Art
It is known that the fundamental frequencies generally encountered during pinging are between 5 and 15 kilohertz for the usual cylinder engines. But it is also necessary, in electronic processing of the signal, to reject other frequencies that correspond to mechanical phenomena due to running of the engine. The characteristic frequencies of both the pinging and spurious phenomena vary from one engine to the next.
For this reason, it is advantageous to have a sensor having a flat frequency response between 1 and 15 kilohertz and beyond, these frequencies being cited by way of nonlimiting example. A good acquisition of the spurious signals makes it possible to filter them electrically in an effective manner.
The prior art of low-cost pinging sensors made from the usual materials machined with low precision relates, on the one hand, to sensors tuned mechanically to a frequency, for example, 6 KHz, which must be adapted to each type of combustion chamber and, on the other hand, to flat frequency response curve sensors. However, experience shows that these latter actually exhibit strong scatterings in the response in at least two frequency zones.
One of the scattering zones is located at high frequencies and results from the fact that, on the one hand, there are numerous rigidities and masses in the kinematic chain and, on the other hand, considering the propagation speeds of sound in the materials used, a quarter-wave is approached in the stacking of parts contributing to the sensitivity of the sensor.
The other zone is located at low frequencies where there are very variable resonances due to the connections on both sides of the piezoelectric element, connections whose mass is not negligible.
Sensors used in the instrumentation field generally are free of these defects but their output signal is very weak, because they use quartz and not ceramic, and their cost is high because of their miniaturization and precision.